Dock levelers may be mounted in loading docks and may be used to bridge a gap between a loading dock and the end of a vehicle parked at the loading dock. For example trucks or tracker trailers may be loaded and unloaded at loading docks with the help of a dock leveler. The dock leveler enables material handling equipment such as a fork lift to move between the dock and the vehicle bed. Because not all vehicle beds are of the same height many dock levelers are configured to pivot up and down in order to adjust and create a bridge between the loading dock and the vehicle bed. Typical dock levelers include a ramp portion. In addition, dock levelers may include a lip mounted at the end of the ramp. When a vehicle backs up to the dock, often the lip is extended and rests directly on the bed of the vehicle. Some dock levelers use the bed of the vehicle as a support for the lip and the ramp so that vehicles, such as fork lifts, material, and operators move between the vehicle bed and the dock.
If a vehicle, whose bed is supporting the dock leveler, were to pull away from the dock, the dock leveler may be unsupported and move rapidly down to an extreme low position under the influence of gravity and strike support structure in the pit of the dock leveler. This condition is known in the industry as free fall. Free fall may have unpleasant consequences particularly when material workers or material handling vehicles such as fork lifts are on the dock leveler during a time when free fall occurs. In order to mitigate the effects of free fall, many dock levelers are equipped with various devices in order to limit or negate the effects of free fall.
One such device used to limit the effects of free fall is a support structure known as a support leg. A dock leveler may have one or more support legs. Often a dock leveler has a pair of support legs. Many support legs are configured so that they support the dock leveler at dock level, (i.e., a position where the ramp is level with the surrounding loading dock). When vehicles back up to a loading dock with the bed of the vehicle located at dock level or above, the dock leveler may be raised, the support legs extended to a supporting position, and the dock leveler lowered until the lip rests on the bed of the vehicle. If the vehicle has a bed located above dock level, the support legs may be slightly above a corresponding support structure configured to support the support legs. Thus, if free fall occurs, the support legs will only permit the dock leveler to fall a limited amount before the support legs engage their support structure.
Alternatively, if a vehicle with a bed located at dock leveler backed up to a dock, the dock leveler will rest on its support legs as well as a lip resting on the bed of the vehicle. Thus, if the vehicle were to pull away from the dock, the dock leveler would not experience any free fall as the support legs would already be supporting the dock leveler.
One problem associated with support legs is that if a vehicle with a bed located below dock level backs up to the dock, the ramp may not be able to lower itself to a below dock position when the support legs are engaged. Thus, the lip portion of the dock leveler must be relied upon to create a ramp between the vehicle bed and the ramp portion of the dock leveler. If the vehicle bed is located too far below dock level the angle of the lip may become steep. Thus inhibiting vehicle traffic between the dock and the vehicle bed. In addition to inhibiting traffic, the steep angle of lip may cause unnecessary wear and tear on the dock leveler, the dock, and the dock leveler support structure as vehicles moving between the vehicle bed and the dock ram into the steep lip. This phenomena of a lip engaging a vehicle bed at a steep angle is known in the industry as stump out.
Stump out can also occur when a vehicle bed is at one level when it first backs up to a dock leveler but as the vehicle is loaded and becomes heavier the vehicle bed may become lower to the ground. As the vehicle bed becomes lower and lower the support legs may engage the support structure thus causing the ramp to be supported by the support legs rather than the vehicle bed. As the vehicle bed becomes lower to the ground due to additional loading of the vehicle the angle between the lip and the vehicle bed may become steeper causing the condition known as stump out.
In order to avoid the condition of stump out many dock levelers include retractable support legs that maintain the ramp in a substantially horizontal position when the ramp is not in use (i.e., when the dock leveler is not engaged with the truck bed). These legs can be retracted for servicing truck beds that are below dock level. Typically a pair of support legs are pivotally attached to the ramp on the lip hinge and extend downwardly to engage a supporting sub frame. These support legs may be spring biased forward toward a supporting position and may be retracted to a non-supporting position by one of several means.
Both manual and automatic mechanisms have been used to retract support legs. Manual support leg retraction mechanisms may require an operator to engage a mechanism, for example, by pulling a chain to retract the support legs as the ramp is being lowered. Automatic support leg retraction mechanisms typically retract the support legs as the lip of the dock leveler is extended. In this way the support leg is retracted when the lip engages a truck. However, retracting the support legs with manual or automatic mechanisms may require additional labor in mating a truck with a dock leveler and may require additional complexity to a dock leveler.
Accordingly, a dock leveler that is able to deal with the problems of free fall and stump out while at the same time reducing the amount of labor and complexity of a dock leveler is desired.